This invention relates to gas discharge displays. More particularly, it relates to alpha-numeric gas discharge displays used in locations exposed to cold temperatures, for example, in gasoline pumps.
A gas discharge display includes a chamber in which is sealed a mixture of inert gases, such as neon and argon, at subatmospheric pressures. Mercury vapor is included within this chamber to impede ions from bombarding the cathode while the display is on. Mercury vapor condenses at about 0.degree. C. inside the sealed subatmospheric chamber. So in applications where the gas discharge display is cooled to temperatures at or below 0.degree. C., condensed mercury will settle onto the cathode. During the operation of the display, the display itself usually produces enough heat to keep the mercury vaporized. Condensation will occur when the display is turned off in a cold environment. The condensed mercury on the cathode surface creates a lump on the surface which decreases the distance between the anode and cathode at that spot. This will cause more current to flow through that spot and will appear as a bright spot in the display. These lumps are called hot spots. The increased current flow through a hot spot can cause the mercury to splatter through the chamber. Mercury settling on the anode blocks the view through the anode creating a black spot in the display.
It is possible for solidification of mercury on the cathode to cause even greater problems. A large build-up of mercury could cause an arc discharge rather than the normal glow discharge. An arc discharge can cause damage to the display surface through the splattering of mercury and the more intense heat. Another problem caused by the condensation of mercury is that there will be less mercury vapor in the chamber to hinder the ion bombardment of the cathode. The increased bombardment will decrease the life span of the cathode, thereby reducing the useful life of the display. To eliminate these problems, it is therefore desirable to heat the gas discharge display prior to turning it on when the ambient temperature is below the condensation point of the mercury.
Where these problems have been recognized, it has been the general practice to include a heating unit behind the display. The heating unit would be separately connected to a power source. In cold weather, prior to switching on the gas discharge display the heating unit is turned on. Problems may be caused if such a separate heating unit is exposed to an adverse environment. For instance, moisture in the air can corrode or rust the heater unit. Condensation of moisture against the glass of the display may cause further problems, such as cracking of the glass. Also, where the gas discharge display is used in a gasoline pump, exposure to gasoline fumes would be undesirable for the heater unit because of the high temperatures and possibility of arcing.
It is the object of the present invention to provide a gas discharge display with a built-in heater. Since the heater will be within the display unit, the resulting product will be more economical, more efficient and easier to connect to a power source.